The application of coating materials to large objects such as automotive and other vehicle bodies has conventionally been accomplished in spray booths having an elongated tunnel-like construction formed with an inlet for the ingress of a vehicle body, a coating application area, a curing or drying area in some designs, and, an outlet for the egress of the vehicle body. In many systems, “conditioned” air, i.e. humidified and filtered air, is introduced by a blower or feed fan into a plenum chamber at the top of the spray booth and then directed downwardly toward the vehicle body moving through the booth. The conditioned air picks up oversprayed coating material within the booth interior and this air entrained oversprayed material is drawn downwardly through the floor or side of the booth by one or more exhaust fans. Filters are provided to capture the oversprayed coating material, and the resulting filtered or clean air is withdrawn from the booth and either exhausted to atmosphere or recirculated within the system for reuse.
The coating material in most common use for vehicles such as automobiles, trucks and the like is a high solids, resinous paint material which contains a relatively high percentage of liquid solvent components to facilitate atomization of the resinous material. The problems attendant to the recovery of oversprayed, resinous paint material have been well documented and present a continuing environmental problem for the coating and finishing industry. See U.S. Pat. No. 4,247,591 to Cobbs, et al. and U.S. Pat. No. 4,553,701 to Rehman, et al.
As disclosed in U.S. Pat. No. 5,078,084 to Shutic, et al., owned by the assignee of this invention, powder coating material has been suggested as an alternative to solvent based liquid paint materials for the coating of large objects such as vehicle bodies. In the practice of powder coating, a powdered resin is applied to the substrate and then the substrate and powder are heated so that the powder melts and when subsequently cooled, forms a solid continuous coating on the substrate. In most powder spraying applications, an electrostatic charge is applied to the sprayed powder which is directed toward a grounded object to be coated so as to increase the quantity of powder which attaches to the substrate and to assist in retaining the powder on the substrate. The application of powder material onto automotive or truck bodies is performed in a spray booth which provides a controlled area wherein oversprayed powder which is not deposited on the vehicle body can be collected. Containment of the oversprayed powder within the booth is aided by an exhaust system which creates a negative pressure within the booth interior and causes the oversprayed powder to be drawn through the booth and into a powder collection and recovery system. The recovered, oversprayed powder can be saved for future use, or is immediately recycled to powder spray guns associated with the powder spray booth.
A number of problems are inherent in coating automotive and other vehicle bodies with powder coating material. Due to the design of vehicle manufacturing facilities, the source of coating material is usually positioned at a remote location from the spray booth, i.e. as much as several hundred feet. Moreover, large quantities of powder coating material, e.g. on the order of 300 pounds per hour and up, must be transferred from the source to the spray booth over this relatively long distance at flow rates such as 1 to 2 pounds per second. Additionally, the powder coating material must be transferred with the appropriate density and particle distribution in order to obtain an acceptable coating of the powder material on the vehicle bodies. The term “density” refers to the relative mixture or ratio of powder-to-air, and the term “particle distribution” refers to the disbursion of powder particles of different sizes within the flow of air entrained powder material to the spray guns associated with the powder spray booth. It has been found that currently available powder coating systems are generally incapable and/or deficient in transporting large quantities of powder material at high flow rates over long distances, while maintaining the desired density and particle distribution.
As noted above, not all of the powder coating material discharged within the powder spray booth adheres to the vehicle bodies moving therethrough. This oversprayed powder material is collected by a powder collection and recovery system at the base of the booth as disclosed, for example, in U.S. Pat. No. 5,078,084 to Shutic, et al. In systems of this type, the powder collection and recovery system includes individual groups or bank of cartridge filters each contained within a series of individual powder collection chambers mounted side-by-side beneath the floor of the spray booth. A single exhaust fan or blower creates a negative pressure within the booth interior, which draws oversprayed, air entrained powder material into each of the individual powder collection chambers where the powder is collected on the walls of the cartridge filters and “clean air” passes therethrough for eventual discharge to atmosphere. Reverse air jets are operated to dislodge the collected powder from the walls of the cartridge filters which then falls to the base of the powder collection chambers where it is removed for collection or recirculation back to the spray guns associated with the powder spray booth.
In high volume applications such as coating automotive vehicle bodies, serviceability of the powder collection and recovery system, and, the application of a uniform negative pressure within the booth interior are of particular concern. It has been found somewhat difficult in certain instances to obtain a uniform negative pressure within the booth interior using a single exhaust or blower fan, which, in turn, adversely affects the efficiency with which the powder coating material can be collected and also can disrupt the pattern of powder coating material discharged from the spray guns onto the vehicle bodies moving through the booth. There has also been a need in systems of this type to improve the serviceability of the reverse air jet valves and cartridge filters contained within each powder recovery chamber.
An additional problem with powder coating systems of the type described above involves recovery of oversprayed powder for recirculation back to the spray guns associated with the powder spray booth. Virgin powder coating material contains a wide particle size distribution, i.e. it includes powder particles which vary substantially in size. The larger powder particles tend to more readily adhere to an object to be coated within the spray booth because they receive a higher electrostatic charge due to their size than smaller particles, and because larger, heavier particles have more momentum than smaller particles when discharged from a spray gun toward an object to be coated. As a result, the oversprayed powder which does not adhere to the object and is collected for recirculation back to the spray guns contains a proportionately greater percentage of smaller particles than the virgin powder since a greater percentage of larger particles in comparison to smaller particles have adhered to the object.
It has been found that the stability of operation of a powder coating system is dependent, at least in part, on avoiding a buildup or accumulation of “fines,” e.g. particles having a size of less than about 10 microns. The term “stability” as used herein refers to the ability of the system to fluidize, transfer and spray powder coating material without problems created by excessive levels of fines. The presence of excessive levels of fines within the powder coating material can result in poor fluidization of the powder, impact fusion, blinding or clogging of filter cartridges and sieve screens, increased powder buildup on interior surfaces of the powder spray booth and on spray guns, and, poor transfer efficiency. The term “impact fusion” refers to the adherence of a powder particle onto a surface as a result of particle velocity as opposed to electrostatic attraction, and “transfer efficiency” is a measure of the percentage of powder material which adheres to an object compared to the total volume of powder sprayed toward the object.
There is essentially no provision in powder coating systems of the type described above to ensure system operating stability when oversprayed powder material is recirculated back to the spray guns after collection. Although venting units have been employed to remove fines from supply hoppers and the like, such units are of limited effectiveness and cannot be relied upon to control with desired accuracy the level or percentage of fines within a given supply hopper.